Hepatitis C virus (HCV) infection is a major cause of post-transfusion and community acquired hepatitis, and results in significant morbidity due to chronic hepatitis in an estimated 50-80% of cases. Attempts to control infection with antiviral agents have met with limited success, and the need for an effective vaccine to prevent continued spread of this infection remains paramount. Central to the development of such a vaccine is an understanding of the virus-specific immune responses which confer protection. Cytotoxic T lymphocytes (CTL) have been shown to be an important component of protective immunity in a number of viral infections. CTL may also mediate tissue damage in the infected host under certain conditions, and have been postulated to contribute to liver damage in viral hepatitis. Characterization of the CTL response to HCV is an important step in determining the role of these cells in disease pathogenesis. During the initial period of support provided by AI31563, our laboratory established a method to detect HCV-specific, HLA class I restricted CTL among liver infiltrating lymphocytes from persons with chronic HCV hepatitis. Our initial studies indicate that a) both structural and nonstructural proteins of HCV are targets of CTL in infected persons; b) these CTL are predominantly compartmentalized in the liver i persons with chronic HCV hepatitis; c) the CTL response is HLA class I restricted and directed at multiple 8-10 amino acid epitopes, and can be directed at multiple epitopes simultaneously; d) CTL epitopes exist in regions which tolerate sequence variation. We have recently established method for expanding HCV-specific memory T cells from peripheral blood will allow us to evaluate the CTL response not only in persons in different stages of acute and chronic infection, but will also facilitate longitudinal studies of CTL and their relationship to disease course. Specifically, we propose to a) Evaluate liver biopsy-derived lymphocytes and peripheral blood lymphocytes from HCV-seropositive persons with symptomatic and asymptomatic chronic HCV infection for the presence of HCV-specific CTL, and characterize the cells mediating this cytotoxicity as well as the dominant viral antigens and epitopes targeted by this response; b) Determine the magnitude and specificity of the HCV-specific CTL response in persons in different stages of infection, including acute infection, chronic infection before and after treatment with interferon, and those with acute flares of hepatitis in order to determine whether a correlation exists between the HCV-specific CTL response and the course of disease; c) Evaluate the potential role of sequence variation as a means of escape from detection by HCV-specific CTL; and d) Determine the HCV-specific CTL response to autologous viruses in infected persons. These studies will provide a detailed understanding of the role of the host cellular immune response in HCV infection, and should facilitate rational vaccine design as well as help to guide immunotherapeutic interventions.